This invention relates to integrated circuits such as programmable logic device integrated circuits, and more particularly, to integrated circuits with memory element power supplies that can be selectively adjusted to reduce power consumption.
Programmable logic devices are a type of integrated circuit that can be programmed by a user to implement a desired custom logic function. In a typical scenario, a logic designer uses computer-aided design tools to design a custom logic circuit. When the design process is complete, the tools generate configuration data files. The configuration data is loaded into memory elements on the programmable logic devices to configure the devices to perform the desired custom logic function.
Programmable logic device memory elements are often based on random-access-memory (RAM) cells. Because the RAM cells are loaded with configuration data during device programming, the RAM cells are sometimes referred to as configuration memory or configuration random-access-memory cells (CRAM).
During normal operation of a programmable logic device, loaded CRAM cells produce static output signals that are applied to the gates of metal-oxide-semiconductor (MOS) field-effect transistors (e.g., pass transistors). The CRAM output signals turn some transistors on and turn other transistors off. This selective activation of certain transistors on the device customizes the operation of the device so that the device performs its intended function.
The performance of modern integrated circuits such as programmable logic devices is often limited by power consumption considerations. Circuits with poor power efficiency place undesirable demands on system designers. Power supply capacity may need to be increased, thermal management issues may need to be addressed, and circuit designs may need to be altered to accommodate inefficient circuitry.
A number of techniques have been proposed for limiting integrated circuit power consumption including substrate biasing, regulation of core logic power supply voltages, use of negative gate voltages, and source side biasing. While these techniques are useful, they do not solve all power consumption problems.
Due to considerations such as reduced power consumption, it is common to operate integrated circuits such as programmable logic devices using multiple power supply levels. Core logic in the interior of an integrated circuit may be powered using a relatively lower power supply voltage called Vcc-core. Input-output circuitry that is used to interface with external system components may be powered using a relatively higher power supply voltage called Vccio. Intermediate power supply levels may also be used.
CRAM cells on a programmable logic device are powered using a positive power supply voltage Vccpg and a ground supply voltage Vsspg. A CRAM cell on a programmable logic device produces an output signal that is high or low depending on the contents loaded into the cell. A CRAM cell that contains a logic one produces a high output signal at the voltage Vccpg. A CRAM cell that contains a logic zero produces a low output signal at the voltage Vsspg.
Due to the operating characteristics of metal-oxide-semiconductor transistors, the values chosen for the CRAM power supply rails Vccpg and Vsspg affect the performance and power consumption of the transistors that are controlled by the CRAM cells. CRAM cells that are powered using relatively low Vccpg voltages such as those used for powering core logic will not turn on their associated pass transistors as fully as they would if a larger voltage were used. This can adversely affect circuit performance, because transistors that are not fully turned on can have difficulties in overcoming parasitic circuit capacitances and other circuit constraints. Because of the impact the values of Vccpg and Vsspg have on circuit performance, it may be desirable to adjust these power supply voltages. However, power supply adjustments should not adversely affect device reliability, performance, or power consumption.
It would therefore be desirable to be able to provide integrated circuits with adjustable CRAM power supplies that allow performance and power consumption to be optimized without compromising reliability.